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15616 HydrocarbonsFig. 16.17According to this molecular orbital picture each carbon-carbon bond in benzene consists of onefull sigma-bond and half a pi-bond. Because of this reason, the carbon-carbon bond length isequal and benzene shows substitution as well as addition reactions.Quick QuizCriticize the following statement:“Benzene is a mixture of molecules most of which have the structure.”Modern Representation of BenzeneWith the help of molecular orbital behaviour weconclude that benzene has a regular hexagonalstructure with an inscribed circle.A hexagon with alternate double and single bonds.or16.7.4. Resonance, Resonance Energy and StabilizationI- Resonance:“The possibility of different pairing schemes of valence electrons of atom is calledresonance” and the different structures thus arranged are called “Resonance Structures”.Explanation:The resonance is represented by a double-headed arrow e.g. the following different pairingschemes of the fourth valence (the p-electrons) of carbon atoms are possible in benzene. Thisgives the following resonatingstructures of benzene: (a), (b) wereproposed by Kekule and c, d, e wereproposed by Dewar. The stability of amolecule increases with increase in thenumber of its resonance structures.
16 Hydrocarbons157Thus molecule of benzene is chemically quite stable.In fact the structure of benzene is a resonance hybrid of all five structures, (a), (b), (c), (d), and(e) in which the Kekule’s structure (a) and (b) have the larger contribution and Dewar’s structurescontribute a little. Therefore, benzene molecule can be represented by either of the two Kekule’sstructure.The three alternate single and double bonds in the above structure are called conjugatebonds of resonating bonds.Since the structure of benzene is a resonance hybrid, therefore all the C-C bond lengths areequal but different from those in alkanes, alkenes and alkynes. It is intermediate between thosein alkanes and alkenes.II- Resonance Energy:The resonance energy of a compound is ameasure of the extra stability of the conjugatedsystem compared to the corresponding number ofisolated double bonds. This can be calculated fromexperimental measurements.The diagram shows the experimental heats ofhydrogenation, ΔHh, for three molecules, benzene,1,3-cyclohexadiene and cyclohexene. These arerelated in that under appropriate conditions theycan all be reduced to the same product,cyclohexane.The ΔHh for "cyclohexatriene", a hypotheticalmolecule in which the double bonds are assumedto be isolated from each other, is calculated to be 3times the value for cyclohexene. This value reflectsthe energy we could expect to be released from 3 isolated C=C.By comparing this value with the experimental value for benzene, we can conclude that benzeneis 152 kJ or 36 kcal / mol more stable than the hypothetical system. This is the resonanceenergy for benzene.16.8 .5. Reactivity and ReactionsThe image shows the electrostatic potential for benzene. The morered an area is, the higher the electron density and the more blue anarea is, the lower the electron density. Note the nucleophilic characterof the aromatic p system.The reactivity issues can be separated into two types ofreactions: reactions of electrophiles directly on the aromatic ring, andFig. 16.18Fig. 16.19
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Textbook ofChemistryGrade 12
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13 s and p - Block ElementsP [Ne] 3
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15 Organic Compounds15.1 SOURCES OF
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Page 111 and 112: 15 Organic CompoundsNitrileMethyle
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Page 115 and 116: 15 Organic Compounds113C) Detection
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Page 119 and 120: 16 Hydrocarbons11716.1 TYPES OF HYD
Page 121 and 122: 16 Hydrocarbons119orororQuick Quiz1
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Page 125 and 126: 16 Hydrocarbons123(b) Alkanes are n
Page 127 and 128: 16 Hydrocarbons125II. CycloalkanesA
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Page 131 and 132: 16 Hydrocarbons129Activity1. Name t
Page 133 and 134: 16 Hydrocarbons1312. Dehydrohalogen
Page 135 and 136: 16 Hydrocarbons133HydrationAddition
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Page 139 and 140: 16 Hydrocarbons137benzeneconjugated
Page 141 and 142: 16 Hydrocarbons13916.6.2 Carbon-Bas
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Page 145 and 146: 16 Hydrocarbons143No geometrical is
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Page 149 and 150: 16 Hydrocarbons147(b)(c)(d)(e)16.7.
Page 151 and 152: 16 Hydrocarbons149terminal alkyne p
Page 153 and 154: 16 Hydrocarbons151BrominationChlori
Page 155 and 156: 16 Hydrocarbons153CumeneDisubstitut
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Page 161 and 162: 16 Hydrocarbons159(c) Addition of H
Page 163 and 164: 16 Hydrocarbons161Explanation and E
Page 165 and 166: 16 Hydrocarbons163SulfonationThe in
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Page 169 and 170: 16 Hydrocarbons167On the other hand
Page 171 and 172: 16 Hydrocarbons169SOCIETY, TECHNOLO
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Page 191 and 192: 17 Alkyl Halides and Amines189CH 3I
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Page 199 and 200: 17 Alkyl Halides and Amines197Key P
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